. the syrian government has a number of sites containing what may be the largest chemical weapons stockpile in the world. it's made up largely of sarin nerve gas, mustard gas and cyanide. on monday, president obama sounded a warning, amid reports of unusual activity at the weapon sites. >> the world is watching. the use of chemical weapons is and would be totally unacceptable. if you make the tragic mistake of using these weapons there will be consequences and you will be held accountable. >> reporter: assad's regime answered that it would never use such weapons against its own people, but today, at nato headquarters in brussels, secretary of state hillary clinton renewed the warning. >> our concerns are that increasingly desperate assad regime might turn to chemical weapons, or might lose control of them to one of the many groups that are now operating within syria. so, as a part of the absolute unity that we all have on this issue, we have sent an unmistakable message that this would cross a red line. and those responsible would be held to account. >> brown: that was a view shared by u.n.